In mining processes an explosive blast is conducted to fracture an orebody to enable recovery of the broken rock to obtain its associated minerals. Explosive material and a detonator for initiating the explosion are placed into a series of pre-drilled narrow elongate holes (blast holes) in the orebody, the area is cleared of people and equipment, and an explosion is set off.
The optimisation of an explosive blast involves crying to obtain certain physical results from the explosion at minimum cost of explosive, which is a significant expense in many mining operations. The specific positioning of sufficient explosive to achieve a successful blast is a complex task. This task is complicated by a need to: avoid over-fragmentation of the rock; heave (or move) as much ore as possible into an accessible position; avoid over-vibration of the ground, around the area being mined; and to protect the working face (high wall of the orebody) and nearby floor of the area from collapse.
In general the desired physical outcomes of an explosive blast are determined by the initiation time between adjacent blast holes, and by matching the initiation time to the response time of the particular rock. The rock response time is related to the speed which cracks are generated around a blast hole by the stress/strain waves created by initiation of an explosion therein. Very hard rock usually has a fast response time (typically 1.5 milliseconds/metre of rock away from the hole), whereas very soft rock usually has a very slow response time (soft sandstone could be 6 milliseconds/metre of rock away from the hole). Blasting at above or below the actual response time can result in a less than optimal physical rock breakage performance.
Pyrotechnic detonators are typically used to initiate an explosion and can have a scatter (or error) of approximately 5% of the initiation time. More recently electronic detonators have been developed and have a scatter of approximately 0.1% so enable a much more optimal performance. Electronic detonators have a programmable microchip which replaces the traditional pyrotechnic delay element. As well as total timing accuracy, electronic detonators can be timed to initiate at intervals of typically anywhere from 0 to 20,000 ms in 1 ms steps. Such flexibility has provided blast design engineers with the ability to create complex blast timing designs.
In order to optimise the physical results of an explosive blast, blasting engineers usually create blast timing designs based on an assumed position of drill holes and a desired blast outcome. This design is typically done prior to drilling of any blast holes, and, out of necessity, assumes certain characteristics of the rock. Detonators are then programmed to go off at a fixed time and in a set sequence. However during drilling of blast, holes, complications can arise and a hole may not be able to be drilled in a pre-determined location because of varying ground conditions. Thus the pre-determined or designed blast hole pattern and the actual blast hole pattern may be very different. Production constraints mean that operators are under pressure to load, set and fire a blast as quickly as possible. Changes from the original blast design may cause major changes to the timing design, which can be very difficult for operators to implement at the time of blasting. The operators sometimes need to make new timing estimates (milliseconds/metre of rock away from the hole), and replace a pre-timed detonator, but an incorrect new estimate by an operator can result in poor physical rock breakage performance. The alternative method is for an accurate off-site redesign of the timing plan by the blast, engineer, however modifications to the blast once the detonators have been programmed may require several hours for redesigning and reprogramming, which is typically not available in a production situation.